linux-kernel - [BUG][RFC] Miscalculated inflight counter in io

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 25 Oct 2019 12:48:07 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [BUG][RFC] Miscalculated inflight counter in io_uring

In case of IORING_SETUP_SQPOLL | IORING_SETUP_IOPOLL:

@inflight count returned by io_submit_sqes() is the number of entries
picked up from a sqring including already completed/failed. And
completed but not failed requests will be placed into @poll_list.

Then io_sq_thread() tries to poll @inflight events, even though failed
won't appear in @poll_list. Thus, it will think that there are always
something to poll (i.e. @inflight > 0)

There are several issues with this:
1. io_sq_thread won't ever sleep
2. io_sq_thread() may be left running and actively polling even after
user process is destroyed
3. the same goes for mm_struct with all vmas of the user process
TL;DR;
awhile @inflight > 0, io_sq_thread won't put @cur_mm, so locking
recycling of vmas used for rings' mapping, which hold refcount of
io_uring's struct file. Thus, io_uring_release() won't be called, as
well as kthread_{park,stop}(). That's all in case when the user process
haven't unmapped rings.

I'm not sure how to fix it better:
1. try to put failed into poll_list (grabbing mutex).

2. test for zero-inflight case with comparing sq and cq. something like
```
if (nr_polled == 0) {
	lock(comp_lock);
	if (cached_cq_head == cached_sq_tail)
		inflight = 0;
	unlock(comp_lock);
}
```
But that's adds extra spinlock locking in fast-path. And that's unsafe
to use non-cached heads/tails, as it could be maliciously changed by
userspace.

3. Do some counting of failed (probably needs atomic or synchronisation)

4. something else?

-- 
Yours sincerely,
Pavel Begunkov

Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)